1. Field of the Invention
The present invention relates to an image scanning device that can increase a scanning speed while securing its scanning quality.
2. Description of Related Art
Conventionally, the following method of scanning an image on an original by an image scanning device is widely used. Light is illuminated on an original placed on a flatbed or an original being conveyed by a conveying means. A one-dimensional photoelectric converter (e.g., line sensor) arranged in a direction (i.e., main scanning direction) perpendicular to a sub-scanning direction (i.e., a moving direction of a scanning mechanism or a conveying direction of the original) then receives the light reflected from the original.
As the photoelectric converter, CCDs are generally used. When the CCDs capture an image, electric charges are accumulated, and an electric signal corresponding to the accumulated electric charges are outputted as the scanned signal of the image. For such a scanning device, when a resolution of the scanning is increased or when a speed of the scanning is increased, a product of a light quantity and a capturing period of time of the image captured by the CCDs decreases inevitably. Accordingly, there is a tendency where the accumulated electric charges are reduced and an acquired S/N ratio of the scanned signal is degraded.
In the meantime, a demand of a higher speed and a higher resolution of the scanning process has increased. Therefore, a photoelectric converter with a time delay and integration (hereinafter, referred to as “TDI”) scheme may be adopted as a means for scanning an image with a high precision. The photoelectric converter of TDI scheme includes a plurality of one-dimensional photoelectric converters. Each of the photoelectric converters scans the same linear area of an original with a time lag therebetween corresponding to a speed of the sub-scan. The scanned signals of the different photoelectric converters are then combined together, and thereby acquiring doubled scanned signals without reducing the scanning speed while scanning with high precision (for example, refer to WO2003/061271).